The present invention relates to a roller mill for pulverizing a feed material such as granulated slag, cement material, cement clinker, gypsum, or coal, and more particularly to a roller mill having a side feed type of feed inlet chute.
There are various types of pulverizers for pulverizing a feed material such as granulated slag, cement material, cement clinker, gypsum, or coal. Of the known various types of pulverizers, a roller mill has been widely used in recent years because of its pulverizing efficiency superior to that of a ball mill. While various types of roller mills exist, they are generally classified into a center feed type and a side feed type from the viewpoint of arrangement of a feed inlet chute.
An example of the center feed type of roller mill is shown in FIG. 7 in vertical section. As shown in FIG. 7, a feed inlet chute 70 extends vertically through an upper central portion of a housing 1. A feed material is supplied through the feed inlet chute 70 forming a vertical passage onto a rotating table 4 along its vertical axis of rotation.
On the other hand, an example of the side feed type of roller mill is shown in FIG. 8A in vertical section. As shown in FIG. 8A, a feed inlet chute 80 extends obliquely downwardly through an upper side wall of a housing 1 in such a manner that the lower end of the feed inlet chute 80 is exposed over a rotating table 4. A feed material is supplied through the feed inlet chute 80 forming an inclined passage onto the rotating table 4 along a line intersecting its vertical axis of rotation.
The general operation of these types of roller mills will be described with reference to FIGS. 7 and 8A. A feed material is supplied through the feed inlet chute 70 or 80 onto the rotating table 4 which is rotated about its vertical axis of rotation by a speed reducer 3 driven by a motor 2. The feed material supplied onto the rotating table 4 is moved to an outer peripheral portion of the rotating table 4 by a centrifugal force generated by the rotation of the rotating table 4. The feed material moved to the outer peripheral portion of the rotating table 4 is pulverized by a plurality of pulverizing rollers 5 rotating in pressure contact with the upper surface of the rotating table 4 at the outer peripheral portion thereof. Each pulverizing roller 5 is pressed on the upper surface of the rotating table 4 by a hydraulic cylinder 7 through a swing arm 6.
The feed material thus pulverized is raised by a heated gas flow injected from a plurality of injection nozzles 8 provided around the rotating table 4, and is then selectively separated by a separator 9 provided at an upper portion in the casing 1, so that fine powder having a given size or less obtained by pulverizing the feed material is allowed to pass through the separator 9 and is then ejected from an outlet opening 1a formed at an upper side portion of the casing 1. On the other hand, relatively coarse powder not allowed to pass through the separator 9 falls on the rotating table 4, and is subjected to pulverization again.
In general, however, the feed material to be pulverized by the roller mill mostly contains fine powder having a high viscosity, and sometimes contains moisture as in granulated slag. Accordingly, such fine powder of the feed material is deposited on the inner surface of the feed inlet chute during passage through the chute, resulting in the formation of a deposited layer of the fine powder strongly sticking to the inner surface of the chute. If this deposit layer is allowed to accumulate, it grows to narrow the passage of the chute through which the feed material can pass. As a result, normal supply of the feed material into the roller mill is hindered by the growth of the deposit layer. Finally, the passage of the chute is choked by the deposit layer resulting in the fatal condition that the feed material can no longer be supplied into the roller mill.
To cope with this problem, various cleaning devices for removing the deposit layer formed on the inner surface of the feed inlet chute at a suitable time have been proposed especially for the center feed type of roller mill. For example, there has been proposed in Japanese Patent Laid-open Publication Nos. 4-145958, 4-176344, and 4-200656 a cleaning device having a jig adapted to be moved vertically inside the vertical feed inlet chute for scraping off the deposit layer.
In the center feed type of roller mill, the feed inlet chute is vertically provided at the top of the housing of the roller mill. Accordingly, auxiliary facilities for carrying the feed material to a high position over the top of the housing become large in size. Further, it is necessary to concentrically arrange the feed inlet chute and the separator. Thus, the roller mill as a whole becomes very complicated in structure with the disadvantages such that the manufacturing cost for the roller mill as a whole, the number of man-hours for the checking and replacement of the feed inlet chute, etc. are high compared to those for the side feed type of roller mill.
The above-mentioned cleaning devices for the center feed type of roller mill as conventionally proposed have the following problems.
In the cleaning device described in Japanese Patent Laid-open Publication No. 4-145958, it is necessary to detach and attach the cleaning device every time the cleaning of the feed inlet chute is carried out. Further, in detaching and attaching the cleaning device and in actually cleaning the feed inlet chute, the operation of the roller mill must be stopped.
In the cleaning device described in Japanese Patent Laid-open Publication No. 4-176344, the cleaning device is always installed in the feed inlet chute in such a manner as not to hinder the pass of a feed material through the chute. Accordingly, it is unnecessary to detach and attach the cleaning device in carrying out the cleaning of the chute, and the chute can be cleaned without stopping the operation of the roller mill. However, when the chute is cleaned by the cleaning device, the fine powder of the feed material is deposited to the cleaning device, so that it is necessary to clean the cleaning device itself and stop the operation of the roller mill when carrying out the cleaning of the cleaning device. Further, the cleaning device cannot be stored outside the feed inlet chute during the operation of the roller mill. As a result, the cleaning device is always exposed to a flow of feed material in the chute, thus promoting the deposition of the feed material to the cleaning device.
In the cleaning device described in Japanese Patent Laid-open Publication No. 4-200656, the feed material chute has a two-way structure to form a space through which a feed material does not pass, so that the cleaning device is stored in this space at any time other than during the cleaning of the feed inlet chute, thereby greatly solving the above problem. However, there still remains the problem that the operation of the roller mill must be stopped when cleaning the cleaning device after the feed inlet chute has been cleaned by the cleaning device.
On the other hand, in the side feed type of roller mill, the deposition of a feed material on the inner surface of the feed inlet chute occurs more readily than that in the center feed type of roller mill, because the feed inlet chute is inclined. Further, since the feed material glides and falls along a circumferential bottom portion of the inner surface of the chute, the circumferential bottom portion in particular is prone to be partially worn.
To suppress the deposition of the feed material, an inclination angle of the feed inlet chute is set to be high in the conventional side feed type of roller mill. However, the enlargement of the inclination angle of the chute causes the problem that the lower end of the chute does not extend to the central portion of the roller mill. As a result, the feed material cannot be dropped on the center of the rotating table thus causing nonuniform supply of the feed material to each pulverizing roller, thus reducing the pulverizing efficiency.
As shown in FIG. 8B which is a cross section taken along the line A--A in FIG. 8A, the partial wear of the circumferential bottom portion of the inner surface of the chute as mentioned above can be prevented by mounting a liner 81 on the inner surface of the feed inlet chute 80 at a U-shaped bottom portion thereof. However, since the liner 81 is formed of ceramics having a high wear resistance or sheet steel with special hard facing, the manufacturing cost of the liner 81 is relatively high.